The invention relates to disconnect switches for the start winding of an AC motor.
An AC motor has a main winding for running the motor, and a start winding energized when starting the motor from rest and then disconnected at a given motor speed. The fields in the main and start windings are phase shifted, for example by capacitance or inductance or a split phase arrangement, to establish a rotating magnetic field for starting torque.
The present invention arose during development efforts attempting to replace the mechanical centrifugal disconnect switch for the start winding in a capacitor start type AC motor, though the invention is not limited thereto. In FIG. 1, main winding 1 and start winding 2 of an AC motor are connectable to an AC power source 3. Capacitor 4 provides the phase shift for starting torque. When the motor reaches a given threshold speed, switch 5 is opened to disconnect start winding 2 from power source 3.
Various types of switches 5, and controls therefor, are known. In one arrangement, a mechanical switch and centrifugal actuator are mounted on the motor shaft or rotor. At a given threshold speed, centrifugal weights are displaced radially outwardly to open the switch. While this type of actuation has proven useful for its intended purpose, it is nonetheless subject to the problems inherent in any mechanical type actuation system, including limited life, fatigue, friction, vibration, mounting position, contact wear, and so on. Also, the centrifugal switch has a radial extent and blocks axial air flow through the motor, which may impair cooling. Furthermore, the centrifugal switch has a given axial extent, thus requiring extra axial room in the motor, which may be objectionable in various applications.
In another known start winding disconnect system, Hall effect sensors are used to detect RPM to actuate a disconnect switch. This approach may be objectionable because of the requirement of adding an extra element such as a magnet on the motor shaft, and the pick-up coil to sense speed. These extra parts and the assembly required may be cost objectionable.
In another known disconnect system, a timer is started at initial energization of the motor. When the timer times out, the disconnect switch is actuated to disconnect the start winding. This approach is not load or speed sensitive, but rather disconnects the start winding after a preselected time regardless of motor speed. This approach is limited to dedicated applications where the load on the motor is known beforehand, and the delay time set accordingly. If the load on the motor is increased, the motor speed may not be up to the desired threshold at the noted cut-out time. On the other hand, if the load on the motor is decreased, the motor will accelerate faster and full voltage will be applied across the capacitor for a longer time than desired, which in turn may damage the motor and/or capacitor. Capacitor burn-out is a significant problem when reducing the loading of the motor in timed disconnect systems.
Another known approach is to sense current through the main winding and then actuate the disconnect switch at a designated condition. This requires a current sensor such as 6, FIG. 1, in series with the main winding and the start winding, which is objectionable to many manufacturers because of the cost of the extra components and the assembly cost of modifying the circuit and inserting such components in series in the circuit. This approach may also be objectionable due to the extra wattage and heat because current is still flowing through sensor 6 in the run mode after starting.
The present invention addresses and solves the above noted and other problems in a particularly simple and effective electronic control for a start winding disconnect switch. The invention is load and speed sensitive and eliminates the need for extra components on the motor shaft, around the shaft, or in series in the motor circuit. There is no physical modification of the components or the windings.